Properties of gasoline, such as its conductivity or dielectric constant, are often important for operation of a motor vehicle. For example, flexible fuel vehicles are known that are designed to run on gasoline as a fuel or a blend of up to 85% ethanol (E85). Such properties can be used to determine the concentration of ethanol in the gasoline/ethanol blend and can also determine the amount of water mixed in with the fuel. For example, experimental data shows that the fuel dielectric constant is directly proportional to the ethanol concentration but relatively insensitive to water contamination, while fuel conductivity is very sensitive to water concentration Thus, for these applications and others, there is a need for a fuel sensor that precisely measures the complex impedance of the fuel.
Current sensor designs have problems handling small capacitance measurements, requiring a relatively large sensing element to increase the signal-to-noise ratio. Further, instead of separately measuring resistance and capacitance, the designs measure total impedance, requiring a relatively high frequency in the 10-100 MHz range to reduce the conductivity impact. Two excitation frequencies are then needed to complete the measurement, a low frequency for resistance measurements and a high frequency for capacitance measurements.
U.S. Pat. No. 6,693,444 entitled CIRCUIT DESIGN FOR LIQUID PROPERTY SENSOR issued to Lin et al. discloses an improvement to the then-prevailing approaches by providing a single frequency circuit design configured to generate magnitude and phase signals corresponding to the complex impedance of the fuel as shown in FIG. 5. Lin et al. disclose a circuit design that characterizes the entire complex impedance of a fuel (i.e., its total complex conductivity). That is, Lin et al. generate both a magnitude signal indicative of total conductivity, including both real (i.e., resistive) and imaginary (i.e., capacitive) parts, as well as a phase signal indicative of the phase angle between an excitation signal and an induced current through the sensing element. While this approach is effective for determining both the dielectric constant as needed for determining ethanol concentration, as well as conductivity as needed for determining water content, further processing is needed to decompose the magnitude signal into its real and imaginary components parts (i.e., one would need to look at just the imaginary part of the magnitude to determine dielectric constant). Additionally, as can be seen in FIG. 5, the circuit is relatively complex.
However, there are certain configurations in the art where just an ethanol concentration sensing system is needed or desired.